This invention relates to a ballast arrangement for fluorescent tubes and particularly to a two-wire ballast arrangement to supply substantially constant filament voltage to a fluorescent tube supplied by a variable power source for dimming.
Typical fluorescent tubes comprise a sealed cylinder of glass having a heating filament at either end and filled with a gas such as mercury vapor. The filaments are heated by a supplied voltage to cause thermionic emission so that an arc can be struck across the tube causing the gas to radiate. The radiation given off by gases such as mercury is short-wave ultraviolet radiation and thus produces little visible light. In order to provide visible light, the inside of the tube is coated with a suitable phosphor which is activated by the ultraviolet radiation and emits visible light of a color that is characteristic of the particularly phosphor or mixture of phosphors employed to coat the tube. In order to sustain the arc across the tube, the filament voltage must be maintained to a predetermined level which poses a problem when the fluorescent tube is to be used in a light dimming arrangement.
As one answer to this problem, the prior art developed a three-wire system where the filament voltage was separated from the variable current used to dim the fluorescent tube. In such a system, a first wire was used to supply variable current to the fluorescent tube. A second wire was used to supply constant filament voltage to the filaments of the tube. The third wire was used as a return. From a cost of installation standpoint, a two-wire arrangement for dimming fluorescent tubes is preferable.
The prior art has developed two-wire fluorescent tube dimming arrangements but these provide only a limited dimming capability. The problem with the prior art two-wire dimming arrangements is that the filament voltage to the tube is varied as the dimming current supply to the fluorescent tube is varied. Thus, as the current supplied to the fluorescent tube is reduced to dim the tube, the filament voltage is also reduced resulting in stripping the emission-coating from the cathodes, and when the filament voltage has been reduced below the level to sustain the arc across the tube, the tube will extinguish. The present invention extends the dimming range of the tube by supplying substantially constant filament voltage to the tube in a two-wire system.